CD players, DVD players and things like the Apple TV all produce 'line level' audio (as does your iPod when it's near it's maximum volume), and this is what the input on your MacBook is designed to accept.
Microphones produce a TINY electrical signal (to give you an idea, in the most common type of microphone, a dynamic microphone, the electrical current is caused by air moving a tiny coil around an equally tiny magnet), and to get this up to the volume needed by your MacBook's line-in, you need a PreAmp.
http://www.dv247.com/invt/9212/ Behringer make a pretty cheap one, although don't expect anything special. Still, it also depends on what microphone you're using. Anything under around £60 worth and you're better off just buying a new USB-powered mic.